Austenitic stainless steels



Dec. 29, 1970 J. H. DECROIX 3,551,142

' Y AUSTENITIC STAINLESS STEELS Filed Jan. 16, 1967 H/S ATTORNEYS United States Patent 3,551,142 AUSTENITIC STAINLESS STEELS Jean H. Decroix, Albertville, France, assignor to Ugine Kuhlmann, Paris, France, a corporation of France Filed Jan. 16, 1967, Ser. No. 609,503 Claims priority, application France, Dec. 14, 1966, 87,383; Dec. 15, 1966, 87,502; Dec. 16, 1966,

Int. Cl. C22c 39/20 US. Cl. 75-128 8 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to new austenitic stainless steels, having improved properties, especially improved creep strength characteristics at high temperatures, and to articles made from these new steels.

When a metal having a satisfactory creep strength up to approximately 650 C. is required, chrome-nickel steels improved by the addition of a variety of elements are frequently utilized. Some such additions, for example,

titanium and aluminum, have the disadvantage that structural hardening must be made during heat treatment before the article is placed in service. Steels obtained in this manner are also costly and not readily Weldable. Other additives, such as nitrogen and vanadium, cause a hardening precipitation in the course of creepage. Still other additives, such as boron, molybdenum, tungsten, copper, manganese and columbium, render the austenitic matrix more rigid at high temperatures.

All of these steels have creep characteristics that are improved when compared to the normal stainless austenitic steels, but in spite of this fact they do not have sufficient creep strength when intended for working at temperatures exceeding 650 C.

The search for ever higher operating temperatures by certain industries, such as those for generating thermal power (superheater tubes), the manufacture of aircraft jet engines, and the manufacture of internal combustion engines (exhaust valves), has led to the frequent use of very special and very costlyalloys which contain up to 20 percent chromium, nickel and cobalt (super alloys), or even 20 percent chromium, 55 percent nickel, 9 percent molybdenum, and 1 percent cobalt (Hastelloy X) for such purposes, or it has led to covering, certain areas of the articles with alloys such as those referred to as stellites.

The present invention provides new austenitic stainless steels which have creep strength characteristics above 650 C. which are comparable to those of the special alloys referred to above. In addition, this invention provides a special treatment for these new steels which gives additional improvement in the properties of the steels. These new steels are particularly useful for superheater tubes, components of aircraft jet engines, blades for gas turbines,

10 steam turbines, and turboblowers, and valves for internal combustion engines.

Steels within the scope of this invention are stainless austenitic steels having a composition within the following limits stated as percent by weight:

Percent Carbon 0.175 Silicon l.0 Chromium 15-20 Nickel 4-16 Manganese 1-12 Molybdenum 0-4 Tungsten 1-6 Copper 0-4 Nitrogen 0.15 Vanadium (1) Nitrogen-l-vanadium 0.65 Columbium 0-2 Boron 0001-0005 Iron and incidental impurities Balance 1 21.2Xnitrogen.

Within the limits specified above, the steels should contain at least one of the elements molybdenum and copper. The steels preferably have a nitrogen content of at least 0.20 percent.

The preferred composition of steels within the scope of this invention is as follows:

Percent Carbon 0.175 Silicon 1.0 Chromium 15-20 Nickel 4-16 Manganese 1-12 Molybdenum 1.5-3.5 Tungsten 1-6 Nitrogen 0.200.30 Vanadium (1) Nitrogen+vanadium 0.65 Vanadium 0.30-0.40 Columbium 01.0 Boron 0.0010.005 Iron and incidental impurities Balance 1 ZLZXnitrogen.

The improved properties of the steels of this invention result from a combination of novel features in the austenitic steels of this invention. These include the addition of either molybdenum or copper or both, in addition to tungsten in the specified limits, the addition of manganese in the specified limits, the particular nitrogen and vanadium contents and the relationship between them, and the addition of boron in the specified limits.

The relationship between the nitrogen and vanadium contents correspond to a balance between these two elements of addition, a balance that permits the precipitation of nitrides of vanadium N V which contributes to the hardness. The boron content is kept sufliciently low to avoid manufacturing difliculties that a higher content would entail, whereby the steels of the invention can be produced without special precautions.

All of the steels of the present invention have an improved creep strength. The stress levels, causing fracture at the end of 5,000 hours, are set forth in Table I.

TABLE I Temperature C.): Stress (kg/mm?) 650 21 700 15 750 9.5

Elongation at creep fracture is at least equal to 10 percent atthe end of 5,000 hours, and the elastic limit E is at least equal to 32 kg./mm.

The single of the drawing shows a plurality of stress curves in which the stress causing creep fracture in 10,000 hours is plotted as a function of temperatures.

sion when hot. The elastic limit at room temperature is of secondary importance. The preferred composition of such steels are set forth in Table II in percent by weight.

TABLE II In the absence of In the presence of Corrosion when hot corrosion when hot Columbium.

Boron Iron and incidental 1mpurities Balance alance.

In the drawing curves 1 to 5 which are drawn in full lines represent known alloys. All of the alloys represented are intended for use in the manufacture of superheater tubes. Curve 1 represents a steel having a composition of percent chromium, 15 percent nickel, molybdenum, tungsten, nitrogen and columbium. Curve 2 represents a steel having a composition of 16 percent chromium, 10 percent nickel, 6 percent manganese, molybdenum, vanadium and columbium. Curve 3 represents a steel having a composition of 17 percent chromium, 14 percent nickel, molybdenum, copper, columbium and titanium. Curve 4 represents a super alloy having a composition of 20 percent chromium, 20 percent nickel, 20 percent cobalt, molybdenum, tungsten, nitrogen, and columbium. And curve 5 represents a Hastelloy having a composition of 20 percent chromium, 55 percent nickel, 9 percent molybdenum and 1 percent cobalt.

Curve 6 drawn in broken lines represents a steel of this invention intended for use in the manufacture of superheater tubes which will not be subject to corrosion when hot. The steel had a composition within the limits set forth below in Example I.

It is clear from the drawing that while the steels represented by curves 1, 2, 3 and 6 and the super alloy represented by curve 4 are nearly identical with respect of creep strength at 650 C., the super alloy represented by curve 4 and the steel of the present invention represented by curve 6 are distinctly superior as soon as the temperature of utilization rises to 700 to 750 C. While the super alloy represented by curve 4 is slightly superior to the steel of this invention at these temperatures, its cost is also much greater. The Hastelloy represented by curve 5, while still superior at 700 C., is surpassed at 750 C. by the steel of the invention of curve 6, and the steel of this invention is also much less expensive.

The end use for which the steels of this invention are intended may dictate a specific composition within the scope of the general composition set forth above. Certain qualities in addition to improved creep strength may be useful, and therefore, in each instance the composition of steel will be selected that has the desired qualities in addition to the improved creep strength. Such qualities, for instance, may be stress-rupture elongation, resistance to corrosion caused by the ashes of impure fuels or by organic lead salts, and elastic limit.

The following specific examples will illustrate the selection of specific steels coming within the scope of this invention for specific uses.

EXAMPLE I Steels suitable for superheater tubes operating above 650 C. require, in addition to good creep strength, stressrupture elongation, and, on occasion, resistance to corro- EXAMPLE 2 Steels used in the manufacture of components of aircrafts jet engines require a high elastic limit. For such uses the preferred composition of steels within the scope of this invention is as follows:

Percent by weight Carbon 50.130 Silicon l Chromium 15-20 Nickel 7-16 Manganese 1-5 Molybdenum 1-3 Tungsten 24 Copper 0-3 Nitrogen 1 Vanadium 2 Boron 0001-0005 Iron and incidental impurities Balance 1 20.15 preferably 20.20. 2 21.2Xnitrogen with V+Nz$0.05.

EXAMPLE 3 Steels well suited for the manufacture of exhaust valves for internal combustion engines require a high elastic limit and resistance to corrosion in the presence of the combustion products of organic lead salts. For such uses the preferred composition of steels within the scope of this invention is as follows:

Percent by weight It is desirable that the composition contain both tungsten and copper, and possibly molybdenum.

In order to further increase the creep strength of steels of this invention at approximately 650 to 800 C., as Well as to increase their elastic limit, they can be treated by means of a special process which entails work-hardening when hot followed by chilling which brings the metal back to ambient temperature while preserving the crystalline structure obtained by the work hardening. In other words, the final conditions for transformation that avoid total or partial self-recrystallization and self-restoration in the structure of the metal is selected. Further, these conditions are determined for each particular category. Moreover, subsequent treatment and conditions of application should not alter the structure obtained by Work hardening when hot.

Although it is generally a well known fact that workhardening increases the elastic limit when cold, as well as the creep strength of austenitic steel, it is noteworthy that applying such a treatment to the steels of this invention produces an improvement in their qualities at very high temperatures, an improvement that is distinctly greater than that obtained from the same treatment of other austenitic steels intended for the same uses.

The following example illustrates a steel within the scope of the present invention subjected to the special process described above and compares the improvement achieved by work hardening of steels within the scope of this invention with the improvement achieved by work hardening a prior art austenitic steel.

EXAMPLE 4 Two steels intended for the manufacture of movable blades for gas turbines having the compositions set forth in percent by weight Table III. Steel 7 is a steel Within the scope of the present invention While steel 8 is an austenitic steel having tungsten and titanium present.

TABLE III Steel 7 Steel 8 Carbon 0.050 0. 100 Silicon. 0. 60 0.50 Chromium 18 17. Nickel- 14. 5 13. 7 Mangancs 1. 75 1. 6O Molybdenum. 2. 53 Tungsten 3. 42 3. 35 Titanium 0. 625 Nitrogen 0. 21 Vanadium. 0. 35 Boron 0.003 0.0 Iron and incidental impuriti 1 Balance.

For both steels the work hardening treatment when hot consisted of a reduction in cross-section by 20 percent at 850-900 C. Table IV gives the results of the measurement of the elastic limits E (in kg. per mm?) and the stresses (in kg. per mm?) which cause creep fracture at 750 C. at the end of 1,000 hours and 5,000 hours. It also shows the relative increase in these values in each steel due tural hardening, and at 750 C. it has a better creep quality than that of the latter steel in which hardening is no longer stable.

In cases where the steels of the invention are used in the form of sheets, it is desirable to manufacture them according to the process described hereinafter. This method of manufacturing makes it possible to improve still further the elastic limit E of the steels of this invention and their resistance to elongation by creeping in the course of the so-called period of secondary creepage. The sheets obtained will, therefore, be particularly suited to certain applications, such as in gas turbines and jet engines.

According to this process, a steel having a composition within the scope of this invention s rolled when hot. This is terminated by a pass of complete self-recrystallization into grains of controlled dimensions. This is then followed by cold rolling and, finally, a thermal annealing treatment at a temperature equal at the most to 1,050 C. and a stress-relaxation treatment that does no cause complete self-recrystallization of the metal. In each case, depending on the analysis and application of the steel, it is necessary to determine the operational conditions of temperature, rate of reduction, and heating or cooling for each stage of the process. These conditions take into account the application, since it is preferable that the service at high temperatures that is required of the steel does not cause complete recrystallization.

The final hot rolling pass should eliminate the work hardening produced by the preceding passes and should produce in the steel the formation of grains in which dimensions are favorable for the continuation of the process. It is known that in order to obtain rather large grains, this latter pass must be carried out at a rather high temperature with a rather low rate of reduction. The cold rolling operation then causes a new work hardening of the metal. The latter stages are intended to relax the metal Without causing complete recrystallization.

The following example illustrates a steel within the scope of the present invention which may be rolled into sheets and compares the properties of the steel sheet when obtained according to the process described above and when obtained by conventional methods.

EXAMPLE 5 A steel within the scope of this invention suitable for making steel sheets for gas turbines and jet engines has a composition as follows:

TABLE IV (7) Work Work (7) Anhard- Relative (8) Anhard- Relative co nealed ened increase nealed ened increase EU kg./m rn.

At ambient temperature 38 82 +116% 26 52 +100% At 650 0 21 +138% 16 34 +112% Stress of creep fracture at 750 C After 1,000 hours 1s. 5 18 +33% 10 13 +30% After 5,000 hours 10. 2 12. 5 +22% 7. 3 8 +11% to the transition from the annealed to the work-hardened Percent by weight condition. Carbon 0.045 The comparison of the relative increases in the values Silicon 0.55 clearly shows that in steel 7 of this invention the work- Chromium 18.8 hardening treatment brings about an improvement in its Nickel 15.2 properties at high temperatures that is much greater than Manganese 1.6 that which is expected with reference to the results of the Molybdenum 2.47 same treatment of a known austenitic steel 8. Tungsten 3.61 The properties of the work-hardened steel 7 of this ing n 0.20 vention are far superior to those of work-hardened steel Vanadlllm 0.37 8, which allows the former to be used at temperatures and Boron 0.003 Iron and mcidental lmpurities Balance with applied stress that are distinctly higher.

It must be noted that steel 7 of this invention has an elastic limit in tensile test and a creep resistance at 700 One sheet (sheet 9) of this composition was obtained by the conventional method comprising successively a hot C. equal to those of a titanium-aluminum steel with strucrolling pass, an annealing treamtent at 1,100 'C., a cold pass, and an annealing treatment at 1,100 C. A second sheet (sheet was obtained by the method of this invention which comprises a hot rolling pass terminated by a pass at a temperature above 1,000 C. with a reduction in thickness of from 5 to percent, a cold pass with a reduction in thickness of from to percent, and an annealing treatment from a temperature on the order of 970l000 C.

The following Table V shows the results of tests on sheets 9 and 10 giving the values for the elastic limits E (in kg. per mm?) measured at various temperatures, and the values of the elongation (in percent on an initial reference length equal to 5.65 /S, S being the section of the test piece) after deformation by creepage obtained by keeping it at 700 C. for 300 hours under a stress of 16 kg./mm.

It should be noted that the improvement in resistance to creep elongation was obtained without any appreciable change in the limit of creep fracture, and with a decrease in ductility at creep fracture of 40 to 20' percent on the average for a fracture caused at the end of approximately 800 hours. And finally, the presentation of the steel of the invention in sheets obtained in according with the process imparts to it an elastic limit and resistance to creep elongation which are on the same order of magnitude as those of a conventional structurally hardened austenitic steel containing titanium and aluminum.

While the present preferred embodiments of this invention have been described, it may be otherwise embodied within the scope of the appended claims.

I claim:

1. A stainless austenitic steel consisting essentially of up to 0.175 percent carbon, up to 1.0 percent silicon, 15 to 20 percent chromium, 4 to 16 percent nickel, 1 to 12 percent manganese, 0 to 4 percent molybdenum, 1 to 6 percent tungsten, 0 to 4 percent copper, at least 0.15 percent nitrogen, vanadium in an amount at least equal to 1.2 times the amount of nitrogen, the sum of vanadium and nitrogen not exceeding 0.65 percent, 0 to 1 percent columbium, 0.001 to 0.005 percent boron, and the balance iron and incidental impurities.

2. A steel as defined in claim 1 in which the molybdenum content is 1.5 to 3.5 percent, the nitrogen content is 0.20 to 0.30 percent, the vanadium content is 0.30 to 0.40 percent and no copper is present.

3. A stainless austenitic steel having improved creep strength above 650 C., ductility, and corrosion resistance when hot consisting essentially of up to 0.06 percent carbon, up to 1.0 percent silicon, 16 to 20 percent chromium, 6 to 10 percent nickel, 5 to 10 percent manganese, 0 to 2 percent molybdenum, 2 to 4 percent tungsten, 1 to 4 percent copper, at least 0.15 percent nitrogen, vanadium in an amount at least equal to 1.2 times the nitrogen content, the sum of the nitrogen and vanadium content not exceeding 0.65 percent, 0 to 1 percent columbium, 0.001 to 0.005 percent boron and the balance iron and incidental impurities.

4. A stainless austenitic steel as set forth in claim 3 in which the silicon content is at least equal to 0.5 percent.

5. A stainless austenitic steel having improved creep strength above 650 C., ductility, and corrosion resistance when hot consisting essentially of up to 0.060 percent carbon, up to 1 percent silicon, 15 to 19 percent chromiurn, 8 to 16 percent nickel, 1 to 3 percent manganese, 1 to 3 percent molybdenum, 2 to 4 percent tungsten, 0 to 3 percent copper, at least 0.15 percent nitrogen, vanadium in an amount at least equal to 1.2 times the nitrogen content, the sum of the nitrogen and vanadium content not exceeding 0.65 percent, 0 to 1 percent columbium, 0.001 to 0.005 percent boron and the balance iron and incidental impurities.

6. A stainless austenitic steel having high elastic limits consisting essentially of up to 0.13 percent carbon, up to 1.0 percent silicon, 15 to 20 percent chromium, 7 to 16 percent nickel, 1 to 5 percent manganese, 1 to 3 percent molybdenum, 2 to 4 percent tungsten, 0 to 3 percent copper, at least 0.15 percent nitrogen, vanadium in an amount at least equal to 1.2 times the nitrogen content, the sum of the nitrogen and vanadium content not exceeding 0.65 percent, 0.001 to 0.005 percent boron, and the balance iron and incidental impurities.

7. A stainless austenitic steel having high elastic limits and corrosion resistance in the presence of combustion products of organic lead salts consisting essentially of 0.10 to 0.15 percent carbon, up to 0.30 percent silicon,

15 to 20 percent chromium, 4 to 8 percent nickel, 8 to 12 percent manganese, 0 to 2 percent molybdenum, 1 to 3 percent tugnsten, 0 to 3 percent copper, at least 0.15 percent nitrogen, vanadium in an amount at least equal to 1.2 times the nitrogen content, the sum of the nitrogen and vanadium content not exceedig 0.65 percent, 0 to 2 percent columbium, 0.001 to 0.005 percent boron, and the balance iron and incidental impurities.

8. A stainless austenitic steel containing about 0.05 percent carbon, about 0.60 percent silicon, about 18 percent chromium, about 14.5 percent nickel, about 1.75 percent manganese, about 2.53 percent molybdenum, about 3.42 percent tungsten, about 0.21 percent nitrogen, about 0.35 percent vanadium, and about 0.003 percent boron.

References Cited UNITED STATES PATENTS 2,793,113 5/1967 Rait. -128.5UX 2,848,323 8/1958 Harris 75129.9X 3,152,934 1/1964 Lula 75-l28.9UX

HYLAND BIZOT, Primary Examiner US. Cl. X.R. 75125 

